Sites usually hide codes. On the backend, this may track when you requested the form (to ensure you didn't store a ton of these), that the code was generated against the form you're actually submitting (so you can't gather a ton and use them) and to prevent automated signups.
Here, I make it pretty easy since the normally "hidden text box" is totally visible. That said, it works the same as other sites, just pops the hood on how it's done.